Am 14.12.2021 um 15:35 hat Stefan Hajnoczi geschrieben: > The BlockBackend root child can change when aio_poll() is invoked. This > happens when a temporary filter node is removed upon blockjob > completion, for example. > > Functions in block/block-backend.c must be aware of this when using a > blk_bs() pointer across aio_poll() because the BlockDriverState refcnt > may reach 0, resulting in a stale pointer. > > One example is scsi_device_purge_requests(), which calls blk_drain() to > wait for in-flight requests to cancel. If the backup blockjob is active, > then the BlockBackend root child is a temporary filter BDS owned by the > blockjob. The blockjob can complete during bdrv_drained_begin() and the > last reference to the BDS is released when the temporary filter node is > removed. This results in a use-after-free when blk_drain() calls > bdrv_drained_end(bs) on the dangling pointer. > > Explicitly hold a reference to bs across block APIs that invoke > aio_poll(). > > Signed-off-by: Stefan Hajnoczi <[email protected]> > --- > v2: > - Audit block/block-backend.c and fix additional cases > --- > block/block-backend.c | 11 +++++++++++ > 1 file changed, 11 insertions(+) > > diff --git a/block/block-backend.c b/block/block-backend.c > index 12ef80ea17..a40ad7fa92 100644 > --- a/block/block-backend.c > +++ b/block/block-backend.c > @@ -828,10 +828,12 @@ void blk_remove_bs(BlockBackend *blk) > notifier_list_notify(&blk->remove_bs_notifiers, blk); > if (tgm->throttle_state) { > bs = blk_bs(blk); > + bdrv_ref(bs); > bdrv_drained_begin(bs); > throttle_group_detach_aio_context(tgm); > throttle_group_attach_aio_context(tgm, qemu_get_aio_context()); > bdrv_drained_end(bs); > + bdrv_unref(bs); > } > > blk_update_root_state(blk);
This hunk is unnecessary, we still hold a reference that is only given up a few lines down with bdrv_root_unref_child(root). The rest looks good to me, so without this hunk: Reviewed-by: Kevin Wolf <[email protected]>
